Watt Hour Capacity of 27 Deep Cycle battery

I am confused what is being said. I do know that 12v lights get dimmer as voltage drops so amps draw actually goes down too. OTOH inverters do have "amps creep"

What I am not clear on is whether it is being claimed that you have the same Power capacity below the 10hr point of the 20 hr rate, but fewer AH left below the "50%" 10 hr point.

I am also asking how the watt meter measures its watts. If it gets a watt no matter how the amps /volts ratio is, then you are getting a funny number where the more amps you get Peukert is happening but the watts don't care.

As said I am confused.

Measuring watt hours with a kilowatt from the inverter output will be very accurate
Measuring the amp hours at the input of the inverter is accurate if using a meter like the Turnigy which records both amp hours and watt hours

But a dc load of a fixed resistance will not increase amps as volts drop, in fact as volts drop amps will also drop

An inverter will draw more amps to maintain the watts output
A fixed resistance dc load will not

Mexi can give us the details
I believe Batteries tested for amp hours use a variable load designed to lower resistance and maintain amps

ENERGY

What produces more POWER?

1,000 amperes at 12.00 volts

or

900 amperes at 14.00 volts?

But but but but but but it's an AMP and an AMP for an HOUR is an AMP HOUR, right?

Hitch five mules so old they have no teeth to a wagon, or four young feisty mules to the same wagon. How far and how fast you gonna get?

A battery that develops 100 amperes and remains at 11.00 volts is going to turn over an engine a hell of a lot faster than a battery that develops 100 amps at 9.6 volts.

It's the true measure of power. It allows folks to know what the POTENTIAL of the device is so they can MANIPULATE components to take full advantage of it if they wish.

Or maybe a bulb consuming 2.2 amps dimmer than all get out is the same as another bulb consuming 2.2 amps but producing significantly more light. Yes, the same type of bulb, no gimmicks.

MrWizard wrote:
I prefer kWh listings over amp hr listings

Amphr listings are for a constant current draw, But the voltage keeps dropping
20 amps @ 12.7v is more watts than 20 amps @ 11.5v
And the draw down test goes to 10.5v
So while the amps are consistent the power used aka Watts, is not

KWH ratings give a truer rating of total power capacity

This is a puzzle worth some attention.

When using a "watt hour" meter, what are you actually measuring and how? If it uses pure amps and volts to derive the watts, then how can it be more valid than the AH meter?

If voltage is declining and amps are rising, won't the AH meter reflect the higher amps? If Watts don't care how much is amps and how much is volts to make a Watt, how can you tell if capacity is lost from the higher amps draw?

Peukert says capacity is reduced for a higher amps draw. Meanwhile Watts don't care if amps are rising since Watts stay the same if voltage correspondingly drops.

I have done load tests at the 20hr rate where it is assumed that 10 hrs is half way, but there is some confusion where things seem to go non-linear at mid- SOCs when taking SGs near 50% and the decline speeds up. (in time)

So first, if staying above 50% like good little boys and girls, then it should not matter while things are still linear, what weird things happen below 50%.

But we also see "amps creep" on the inverter's draw as voltage declines, sort of what Mr Wiz is talking about wrt constant amps but voltage declines except that the inverter tries to maintain constant Watts.

IMO using WH vs AH may be more accurate, but first there needs to be an agreement on whether the Watts measurement's input is more accurate than the AH's.

There is just something funny about those Watts thingies. :(

deleted

I prefer kWh listings over amp hr listings

Amphr listings are for a constant current draw, But the voltage keeps dropping
20 amps @ 12.7v is more watts than 20 amps @ 11.5v
And the draw down test goes to 10.5v
So while the amps are consistent the power used aka Watts, is not

KWH ratings give a truer rating of total power capacity

Serious information expresses wattage in kWh. It is useful in design applications especially where batteries with different construction are being considered. For instance a calcium / calcium telephone communications power backup system will deliver significantly more wattage when discharging from 100% SOC to 80% SOC.

Time spent at outage, versus watts needed to support equipment will reveal an optimum battery bank size. No maintenance and low self-discharge rates also come into play.

Where wattage could make a difference in RV design would be sizing a dedicated bank of batteries for use with an inverter and high load device like a microwave or convection oven.

Salesman at work.

Buzzword IS "the word".

Something new for the boss or get laid off.

Homeless by Choice wrote:

All I want to do is select a solar panel system that will fully charge my two batteries each day starting with 50% SOC.

I want this for a horizontal panel system for 66 degrees latitude on August 1st. If I can achieve that I will be very happy.

I'll suggest 3 120 watt panels flat. ~4 Kwh/m^2/day flat panel on average for Aug 1st in Fairbanks AL.

120 watt panel is rated ~7-7.5 amp/kwh of Solar Irradiance.

7.5 x 3 panels * 4 (Solar Irradiance) = 90 ah or ~ on G27 battery!!!

Bob Landry wrote:
Watt hours is irrelevant. Amp hours is usually used to describe deep cycle battery capacity. It looks like the one you are referring to is a group 27 since that number stands out. if so, that battery is a little lacking. A group 27 battery has aprox 105 Amp Hrs of capacity. You can take the current consumption of all of your DC loads, multiply by the time it actually runs and figure how long you can run to get down to 50% charge. I'm not a fan of Interstate batteries, The marinas and repair shops around here all sell them and they replace a lot of them on a yearly basis. I personally prefer the no-maintainence batteries..JMHO, YMMV.

Well, we also sell Interstate and have for over 25 years. Interstate is the BEST battery, Period. People that replace usually have NOT maintained them (water level) or have abused them. Maintanance free batteries in a RV are a BIG NO No. They do not work as well as regular lead acid batteries that are NOT maintenance free. I base this statement on 34 years in the RV service industry and have seen the failures when Customers and OEM's have gone the Deep cycle maintenance free installs. We RARELY replace any Interstate battery under warranty. Doug

As I said, the amp hours times volts is simplified for watt hours

But amps times volts = watts.

Thus amp hours times volts = watt hours

I also posted the full formula and explained why the simplified version is valid.

I also mentioned that the speed of discharge makes a big difference... OH, and thanks for that chart, That one I am putting in my reference folder, Thanks.